Film bulk acoustic resonator (hereafter FBAR) is an advanced resonator. As shown in FIG. 1A, a FBAR 10 has a metal layer 11—piezoelectric layer (hereafter PZ layer) 12—metal layer 13 structure formed by thin film semiconductor processes in a controlled environment, the PZ layer 12 being an aluminum nitride or a zinc oxide layer, and the metal layers 11 and 13 being connected with a first electrode 14 and a second electrode 15, respectively. When alternating voltage is applied across the first electrode 14 and the second electrode 15, resonance is generated as a result of expansions and contractions within the aluminum nitride or zinc oxide layer in response to the alternating voltage. Since Surface Acoustic Wave Devices (hereafter SAWs) only employ surface area to produce resonance, FBARs possess superior power handling capacity, especially in high frequency regions, owing to the fact that the bulk of the aluminum nitride or zinc oxide layer is engaged in resonance generation. In high frequency operation, the dimension of SAWs' interdigitated structure has to be greatly reduced, demanding highly sophisticated processes. For high frequency applications such as filters, duplexers and oscillators, FBARs owns superior device characteristics.
FIG. 1B shows an input impedance curve of a FBAR 10 as a function of alternating voltage, fr and fa being resonance frequency and anti-resonance frequency, respectively. When metal layer 11, PZ layer 12 and metal layer 13 are formed as a result of imperfect thin film semiconductor processes, resonance frequency and anti-resonance frequency shifting often occurs, making quality assurance a major challenge.
The following lists a number of prior art tuning methods for FBARs:    1) U.S. Pat. No. 5,873,153 ‘Method of Making Tunable Thin Film Acoustic Resonators’ by Ruby et al. discloses that, constructed on a thin membrane 201, a FBAR 20 (See FIG. 2A) includes bottom and top electrodes 204 and 203, respectively, which sandwich a portion of sheet of PZ material 202. The temperature of the PZ material 202 is controlled by resistive heating elements 205 and 206. The patent reports, by altering the temperature of a FBAR over a range of 200° C., a frequency shift of 4% may be obtained. The method, however, has two deficiencies:            1. The tunable resonance range is limited        2. Resistive heating equals energy consumption        
Referring to FIG. 2B, the patent also takes advantage of reducing a acoustical path 222 in order to achieve increased resonance frequency of a FBAR 22. What it does is to incorporate an additional layer 221, which is a conductor having a high resistivity, on the underside of FBAR 22. In this embodiment of the prior art patent, the fabrication parameters are chosen such that FBAR 22 has a resonance frequency that is slightly below the desired frequency. In post fabrication testing, the frequency is measured as material from layer 221 is evaporated and acoustical path 222 is reduced until the frequency increases to the desired value. The embodiment also has two drawbacks:                3. Resonance tuning only take place in vacuum        4. Requires sophisticated tuning techniques            2) U.S. Pat. No. 6,051,907 ‘Method for Performing On-wafer Tuning of Thin Film bulk Acoustic Wave Resonators (FBARS)’ by Ylilammi illustrates the manner in which the series resonant frequencies exhibited by an FBAR are related to various thickness of an added layer of piezoelectric material that is formed of zinc-oxide (ZnO). The prior patent demonstrates that an additional zinc-oxide layer having a thickness of 149 nm needs to be formed over a selected portion of the FBAR having a series resonant frequency of 994.28 MHz in order to tune the series resonant frequency to a target series resonant frequency of 954.6 MHz. The disadvantage is that a slight variation in thickness can result in considerable change in resonant frequency. Hence, the prior art patent shares the same drawbacks of items 3 and 4:            3. Resonance tuning only take place in vacuum        4. Requires sophisticated tuning techniques            3) U.S. Pat. No. 5,166,646 ‘Integrated Tunable Resonators for use in Oscillators and Filters’ by Avanic et al. illustrates a resonator 31, comprising a FBAR 30 and a Voltage Variable Capacitor or VVC 32. The VVC 32 comprises a heavily doped semiconductor layer 322 on a common substrate 321, an upper electrode 324 on an insulator layer 206 and a lower electrode 325 on the underside of VVC 32. A reverse bias voltage applied between electrodes 324 and 325 results in the formation of a depletion region 326. The variations in the width of this depletion region 326 provide a large variation in the tuning capacitor coupled to the FBAR 30. It is this characteristic of the VVC 32 that renders the FBAR 30 tunable. By varying reverse bias voltage, the operating frequency of the FBAR 30 and hence the resonator 31 may be changed resulting in a tunable resonator. As the reverse biased voltage applied between 324 and 325 is varied so will the reactance of the VVC 32 and hence the operating frequency of the resonator 31. The tunable resonator 31, however, has two deficiencies:            1. The VVC 32 is a varactor, which is an active component, and will thus increase the overall noise of the tunable resonator 31        2. The application of semiconductor characteristics of the common substrate 321 prevents usage of non-semiconductor substrates        
The present invention has been accomplished with a view to overcoming the above-mentioned problems experienced with conventional technologies.